std:: conjunction

Forms the logical conjunction of the type traits B... , effectively performing a logical AND on the sequence of traits.

The specialization std :: conjunction < B1, ..., BN > has a public and unambiguous base that is

• if sizeof... ( B ) == 0 , std:: true_type ; otherwise
• the first type Bi in B1, ..., BN for which bool ( Bi :: value ) == false , or BN if there is no such type.

The member names of the base class, other than conjunction and operator= , are not hidden and are unambiguously available in conjunction .

Conjunction is short-circuiting: if there is a template type argument Bi with bool ( Bi :: value ) == false , then instantiating conjunction < B1, ..., BN > :: value does not require the instantiation of Bj :: value for j > i .

If the program adds specializations for std::conjunction or std::conjunction_v , the behavior is undefined.

Template parameters

Helper variable template

Possible implementation

template<class...>
struct conjunction : std::true_type {};
 
template<class B1>
struct conjunction<B1> : B1 {};
 
template<class B1, class... Bn>
struct conjunction<B1, Bn...>
    : std::conditional_t<bool(B1::value), conjunction<Bn...>, B1> {};

Notes

A specialization of conjunction does not necessarily inherit from either std:: true_type or std:: false_type : it simply inherits from the first B whose ::value , explicitly converted to bool , is false , or from the very last B when all of them convert to true . For example, std :: conjunction < std:: integral_constant < int , 2 > , std:: integral_constant < int , 4 >> :: value is 4 .

The short-circuit instantiation differentiates conjunction from fold expressions : a fold expression, like ( ... && Bs :: value ) , instantiates every B in Bs , while std :: conjunction_v < Bs... > stops instantiation once the value can be determined. This is particularly useful if the later type is expensive to instantiate or can cause a hard error when instantiated with the wrong type.

Example

#include <iostream>
#include <type_traits>
 
// func is enabled if all Ts... have the same type as T
template<typename T, typename... Ts>
std::enable_if_t<std::conjunction_v<std::is_same<T, Ts>...>>
func(T, Ts...)
{
    std::cout << "All types in pack are the same.\n";
}
 
// otherwise
template<typename T, typename... Ts>
std::enable_if_t<!std::conjunction_v<std::is_same<T, Ts>...>>
func(T, Ts...)
{
    std::cout << "Not all types in pack are the same.\n";
}
 
template<typename T, typename... Ts>
constexpr bool all_types_are_same = std::conjunction_v<std::is_same<T, Ts>...>;
 
static_assert(all_types_are_same<int, int, int>);
static_assert(not all_types_are_same<int, int&, int>);
 
int main()
{
    func(1, 2, 3);
    func(1, 2, "hello!");
}

All types in pack are the same.
Not all types in pack are the same.

See also